The way this workflow was envisioned, is to emulate the real-life packing process. That is why we expect users to fill the actual content by scanning, in the warehouse: It represents equipment that is actually inside, in real life. It is also good to know that stock is calculated based on default content.
We decided not to block users in this flow: If they scan items into a combination, they are by definition inside. In the future we might consider giving more warnings, if adding items to a combination would cause an inconsistency in stock levels.
Please let us know if you feel this would improve the system.